Liquid crystal display devices are remarkably widespread as image display devices for personal computers and various other monitors. In general, such a liquid crystal display device comprises an illuminating backlight (a planar light source) behind a liquid crystal display panel, the light irradiating the liquid crystal panel which provides a certain spread of even brightness. An image is thus formed on the liquid crystal display panel.
Such a liquid crystal display device includes the aforementioned liquid crystal display panel (which is typically composed of two glass substrates and a liquid crystal material sealed therebetween), a printed circuit substrate for driving the liquid crystal material on the display panel, the described backlight unit disposed behind the liquid crystal display panel, and an exterior frame for holding (and covering) these components. In a thin-film transistor (TFT) liquid crystal display device, one of the glass substrates constituting the display panel includes an array substrate, and the other glass substrate includes a color filter substrate. On the array substrate (in addition to TFTs as driver elements of the liquid crystal material, display electrodes, and signal lines) are formed extraction electrodes for electrical connection to the above-mentioned printed circuit substrate and the like. Since the TFTs are arranged on the glass substrate, the glass substrate is referred to as an array substrate. On the color filter substrate (in addition to color filters) are formed common electrodes, black matrix and the like.
The printed circuit substrate is generally connected to (or mounted on) the extraction electrodes via a tape-automated bonding (TAB) tape carrier (hereinafter simply referred to as a “TAB”) formed on the array substrate. Input lead conductors of the TAB are connected to corresponding electrodes of the printed circuit substrate by solder, for example. Meanwhile, output lead conductors of the TAB are connected to corresponding extraction electrodes of the array substrate. An anisotropic conductive film (ACF) or an anisotropic conductive paste (ACP) has been conventionally used to connect the output lead conductors of the TAB to the corresponding extraction electrodes of the array substrate.
Besides assembly using TAB, another assembly technique called chip on glass (COG) may be used. COG is a technique to bond an IC silicon chip (hereinafter referred to as a “silicon chip”) onto the array substrate with the ACF or ACP.
The ACF or the ACP (hereinafter collectively referred to as the “ACF”) comprises a resin material as an adhesive with particles composed of a conductive material dispersed therein. There are two types of ACF, namely, thermoplastic ACF that uses thermoplastic resin as an adhesive and thermosetting ACF that uses thermosetting resin as an adhesive.
Thermal pressurization (involving heating and pressurizing) is commonly used in both thermoplastic ACF and thermosetting ACF bonding techniques. An example of a bonding technique using thermal pressurization will be described with reference to FIG. 6. FIG. 6 shows an apparatus for bonding a silicon chip 121 onto an array substrate 123 of a liquid crystal display panel 120 with ACF 124. A color filter 125 and a polarizer 126 are also parts of panel 120. The bonding apparatus is comprised of a heater tool 111 (having an internal heater (not shown)) and a back-up block 116.
When silicon chip 121 is bonded onto array substrate 123, as shown in FIG. 6, heater tool 111 is heated while sandwiching array substrate 123, silicon chip 121 and ACF 124 between heater tool 111 and back-up block 116. The heat from heater tool 111 is conducted to ACF 124 via silicon chip 121. ACF 124 is therefore heated and cured by this thermal conduction. Another example of a heating method is a pulse heating method, in which heat loss of metal generated by application of a large electric current with a low-frequency pulse to heater tool 111 is utilized to instantly generate heat. The pulse heating method has an advantage of the large freedom of temperature and pressure profiles. Such bonding methods incur various problems, particularly when applied to a large-sized liquid crystal display panel requiring a narrow pitch and a narrow frame. One such problem is an occurrence of uneven assembly caused by a difference in contraction between the array substrate abutting on the ACF and a TAB or silicon chip after thermal expansion which occurs when assembling such TABs made of polyimide and the like and objects for assembly composed of silicon chips and the like. The uneven assembly occurs in part due to the adhesive power of the ACF.
Such occurrence of unevenness becomes particularly evident upon assembling a silicon chip because of its high rigidity compared to that of the typically flexible TAB. This is a major factor affecting assembling of silicon chips for use with large-sized, high-definition liquid crystal display panels. In the case of assembling the TAB component of the assembly, the occurrence of uneven assembly is not as significant because polyimide has sufficiently low rigidity compared to that of glass.
FIGS. 7A to 7C are views illustrating formation of an uneven assembly.
In FIG. 7A, heater tool 111 is a heater for heating the TAB or silicon chip 121 by thermal conduction. Silicon chip 121 is placed on array substrate 123 via thermosetting ACF 124. If the temperature necessary for curing ACF 124 is 210 degrees Celsius, for example, then a heating temperature of heating tool 111 should be set at about 250 degrees Celsius. In this example, a temperature of a bottom surface of array substrate 123 reaches about 70 degrees Celsius. That is, a substantial heat gradient arises in a direction from silicon chip 121 to array substrate 123 (depicted by the downward arrow in FIG. 7A). FIG. 7B is a view showing a state of a cooling process after ACF 124 has been heated. Chip 121 contracts when its temperature drops, in a direction illustrated by the two upper arrows in the upper part of FIG. 7B. Array substrate 123 contracts similarly in a direction illustrated by the two lower arrows in the lower part of FIG. 7B. It should be noted that lengths of these arrows also represent magnitude of contraction, those relating to chip 121 indicating greater contraction than those of array substrate 123.
FIG. 7C represents a state in which ACF 124 is completely cured and that silicon chip 121 is thus firmly bonded to array substrate 123. In this event, since a heating temperature of array substrate 123 is lower than a heating temperature of silicon chip 121, silicon chip 121 shows greater contraction. Accordingly, as shown in FIG. 7C, silicon chip 121 and array substrate 123 bound by ACF 124 are all distorted (curved). In FIG. 7C, the resulting camber has chip 121 on its inner curve, due to the greater contraction of chip 121 compared to array substrate 123. As array substrate 123 becomes thinner in response to a demand for thinner liquid crystal display devices in the future, or in the event that low-rigidity glass is used for array substrate 123, such distortion (camber occurrence) may pose a major assembly problem.
A second problem is that color filter 125 (FIG. 6) and the like may be damaged by heat from heater tool 111 should the heater tool come too close to this component during heating. One example of a temperature required for curing ACF 124 ranges from approximately 170 degrees Celsius to 230 degrees Celsius; however, as indicated above, the heating temperature of heater tool 111 is higher (e.g., in the example, about 30 to 40 degrees Celsius).
Accordingly, substantial heat may be applied to the liquid crystal material, adhesive, color filter pigments, polarizers and the like of the liquid crystal display panel. Such heat, as understood, presents a risk of deforming the liquid crystal material and the seal adhesive.
Japanese Patent No. 2568853 discloses a technology for curing a thermoplastic ACF by irradiation of infrared rays. According to this patent, heat for curing the ACF is generated by irradiation of the infrared rays onto the ACF. This heating presents an opportunity to avoid some of the above-described problems. However, the technology does not propose any effective measures to prevent occurrence of the cambers caused by the different temperature gradients.
Japanese Patent Laid-Open Publication Hei 5(1993)-206220 also discloses a technology for heating and curing the ACF by irradiation of infrared rays. However, according to the technology disclosed in this Laid-Open Publication, a tape carrier package (TCP), which is an object of bonding using the ACF, is preheated and then irradiated by the infrared rays, heating the ACF using the heat generated thereabout. Accordingly, this approach also presents a problem due to the temperature gradients. Moreover, this technology requires coating the TCP with a black carbon material for facilitating absorption of the infrared rays and blending of a similar coating material into the ACF for facilitating absorption of the infrared rays. Such requirements add cost to the assembly process (and resulting product) because these require the added coating step and/or blending of the coating material.